Stephen Hawking: 'The universe has many alternative histories, but which is real?'

From a speech by the Lucasian Professor of Mathematics at Cambridge University, to the Prince of Asturias Foundation in Oviedo, Spain

From a speech by the Lucasian Professor of Mathematics at Cambridge University, to the Prince of Asturias Foundation in Oviedo, Spain

I am here to talk about Running History Backwards. We think of history as a progression forward in time. One event causes another, and so on. The principle of scientific determinism was formulated by the French scientist the Marquis de Laplace in the early 19th century. Given a knowledge of the state of the universe at one time, the laws of science uniquely predict its future evolution.

Scientific determinism also works backwards. Given the state of the universe at one time, there is a unique previous history that led to it. So why don't we tell history backwards? The reason is that, because we usually don't have a complete knowledge of the state of a system, we are more likely to be able to predict its future evolution than its history.

The bottom-up approach to cosmology, in which one runs the history of the universe forward in time, would be appropriate if one knew that the universe was set going in a particular way in the past. However, in the absence of such knowledge, it is better to work from the top down, by tracing backwards from the final state the histories that contribute to the sum over histories. This means that the histories of the universe depend on what is being measured, contrary to the usual idea that the universe has an objective, observer- independent history.

What does it mean to say that the universe has many alternative histories? Which is the real history of the universe? To answer such profound philosophical questions, I think one must adopt the positivist approach of Karl Popper and others. in this, a theory is just a mathematical model to describe the observations. It has no claim to reality, whatever that may be. Two very different models may describe the same observations. Both theories are equally valid, and neither can be said to be more real than the other.

The results are disappointing for those who hoped that the ultimate theory would predict everyday physics. We live in the anthropically allowed region in which life is possible, but I think we might have chosen a better location.